Brittle as iron strongbox (4)
I believe the answer is:
safe
'strongbox' is the definition.
(I know that safe is a type of strongbox)
'brittle as iron' is the wordplay.
'brittle as' becomes 'sa' (I can't explain this - if you can you should give a lot more credence to this answer).
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'sa'+'fe'='SAFE'
Can you help me to learn more?
(Other definitions for safe that I've seen before include "Secure and protected" , "Metal case for storing valuables" , "Secure in metal box?" , "Secure and free from danger" , "Uninjured, secure" .)